The present invention relates generally to production of high vacua and more particularly to improved organosilicon fluids for use in vapor booster diffusion pumps.
Vapor booster pumps are a variety of diffusion pumps specifically constructed for use in such diverse technologies as vacuum metallurgy, food processing, distillation, environmental simulation, vacuum coating, cathode ray tube evacuation, and the like, where it is desired that large quantities of gas be pumped at vacuums in a range somewhat lower than those employed [i.e., 10.sup.-6 to 10.sup.-8 torr. (untrapped) and 10.sup.-10 to 10.sup.-11 (trapped)] in diffusion pumps of a general construction. Manufacturers of such pumps ordinarily specify that they be operated with fluids capable of developing ultimate vacuums "better than 10.sup.-4 torr." or at least within the range of 1.times.10.sup.-4 to 5.times.10.sup.-4 torr. Additional characteristics desired in fluids for use in vapor booster pumps include: resistance to thermal oxidation (thus avoiding the need for pump cooling before the vacuum is released); resistance to decomposition into chemical byproducts which reduce ultimate vacuum (thus avoiding the need for frequent fluid replacement); and, nonreactivity with metal parts, elastomeric seals, hydrogen and carbon monoxide gases. The ongoing search for fluids possessing the above-noted characteristics has continued over the years.
The use of organopolysiloxanes and silahydrocarbons as diffusion pump fluids has been known since approximately 1945. Among the compounds of interest to the background of the present invention are phenylmethylsiloxanes and particularly low molecular weight ogligomers. U.S. Pat. No. 2,530,356 discloses diffusion pump fluid usages for a large number of such compounds having a boiling point of 250.degree.-500.degree. C. at atmospheric pressure. U.S. Pat. No. 2,567,110, as another example, discloses the preparation of a variety of phenylmethyl siloxanes, including mixtures of siloxanes of the formula; ##STR1## wherein n has an average value of 4. Such fluids were noted to have a boiling point greater than 200.degree. C. at 1 mm.Hg. and were therefore proposed as having utility as diffusion pump fluids. The use as diffusion pump fields of other, lower molecular weight, phenylmethylsiloxanes as diffusion pump fluids is noted in U.S. Pat. No. 2,890,234. That reference discloses the preparation and use of both 1,3,3,5-tetraphenyl-1,1,5,5-tetramethyltrisiloxane and 1,3,5-triphenyl-1,1,3,5,5-pentamethyltrisiloxane as diffusion pump fluids.
Despite the above-noted interest in relatively low molecular weight phenylmethylsiloxanes (hexasiloxanes in U.S. Pat. No. 2,567,110 and trisiloxanes in U.S. Pat. No. 2,890,234), proposals for use of phenylmethyltetrasiloxanes and phenylmethylpentasiloxanes are conspicuously absent from the art pertaining to diffusion pump fluids. This is likely due in part to the fact that their reported boiling points are rather low. Low boiling points generally indicate that the particular fluid is too volatile and therefore too difficult to condense at high vapor operation. More significantly, the reported vapor pressures of these siloxanes are such as would make them appear to be of little use in diffusion pumps. V.E. Ditsent, et al., reporting on the vapor pressures of alpha-omega hexamethylpolymethylphenylsiloxanes in the Russian Journal of Physical Chemistry, 45 (6), (1971), reported the A and B coefficients for the vapor pressure of nonamethyl-3,5,7-triphenylpentasiloxane as 9.1545 and 4124, respectively. The calculated ultimate vacuum at 25.degree. C. for such a compound, using the equation log P=A-B/T generates, EQU log P=9.1545-(4124/298.2)=-4.6751 EQU P=0.000021 or 2.11.times.10.sup.-5 torr.
The A and B coefficients for the vapor pressure of octamethyl-3,5-diphenyltetrasiloxane reported by Ditsent, et al, are 8.8435 and 3543 respectively. Again applying log P=A-B/T for calculation of the ultimate pressure, there is generated EQU log P=8,8435-(3543/298.2)=3.03779 EQU P=0.00092 or 9.2.times.10.sup.-4 torr.
Such an ultimate vacuum value is nearly one order of mangitude removed from manufacturers' recommendations of 10.sup.-4 torr. for vapor booster pump fluids and indicates that the tetrasiloxane would be useless in vapor booster pumps.